Vascular plants are plants in the Kingdom Plantae that have specialized tissues for conducting water. Vascular plants include the ferns, clubmosses, horsetails, flowering plants (angiosperms), and conifers, and other gymnosperms. Scientific names are Tracheophyta and Tracheobionta, but neither is very widely used. Nonvascular plants include lineages both in Plantae (mosses, hornworts, and liverworts) and members of other kingdoms (the various algae).
Water transport happens in either xylem or phloem: the xylem carries water and inorganic solutes upward toward the leaves from the roots, while phloem carries organic solutes throughout the plant.
An analogy has been commonly drawn between the vascular system of plants and the circulatory system of the human body. Just as veins and arteries play different, but unified, roles in transporting essential elements via blood from one part of the human body to another, the phloem and xylem tissues consist of tubes that transport essential fluids and nutrients in sap, from one part of the plant to another.
Vascular plants are named from the latin word vasculum, meaning "vessel" or "duct." The evolution of this vascular tissue allowed for an early dominance of these plants on land (first appearing 430 million years ago, during the Silurian period), giving them the ability to transport water and dissolved minerals through specialized strands of elongated cells that run from the plant root to the tips of the leaves.
Vascular plants evolved several important features:
- Vascular plants have water-carrying tissues, enabling the plants to become a larger size. Non-vascular plants lack these and are restricted to relatively small sizes.
- In vascular plants, the principal generation phase is the large, dominant, nutritionally-independent sporophyte, which is diploid with two sets of chromosomes per cell. In non-vascular plants, the principal generation phase is often the gametophyte, which is haploid with one set of chromosomes per cell.
- Specialized leaves, stems and roots
- Vascular plants have cuticles and stomata to prevent dessication and facilitate gas exchange, respectively.
- Dessication tolerant seeds. Non-vascular plants require water for fertilization, whereas seeds are dessication tolerant and can remain dormant until conditions are right for reproduction.
Vascular plant evolution and basic types
Early vascular plants only developed by primary growth, in which the plants grew through cell division of the plant body. These early plants did not have differentiated stems, leaves, or roots. They did, however, contain vascular cylinders, which perform the same role as the xylem and phloem in vascular plants today.
Secondary growth developed early (the Devonian period, 380 million years ago) in the evolution of vascular plants, which allowed for cell division to take place in the active regions of the plant's periphery. This was an important evolutionary trait that allowed for plants to grow in diameter and form tree-like growth. During this time, vascular plants were able to expand greatly in size.
Seed PlantsMain articles: Gymnosperm, Angiosperm
Seeds developed in more advance vascular plants about 360 million years ago, and are now classified as either angiosperms or gymnosperms, and collectively called the seed plants. In these plants, the gametophytes are highly reduced. The seed contains an embryo, which is protected by a hard outer coating. The embryo's, or sporophyte's, growth has been temporarily arrested, and seeds can remain dormant until appropriate reproductive conditions prevail. Seeds allow for rooted plants to disperse and increase their range. Because of its resistance to predation, drought, and other factors, seed development was instrumental in the dominance of seed plants on land.
The seed-bearing vascular plants are grouped under the superdivision Spermatophyta. The phyla include Pinophyta (conifers), Cycadophyta (cycads), Ginkgophyta (ginkgoes), Gnetophyta (gnetophytes), and Magnoliophyta (flowering plants or angiosperms).
These groups are discussed in more detail in the gymnosperm and angiosperm articles.
Seedless plants developed before the seed plants and include four phyla of living vascular plants, including Pteridophyta, Equisetophyta (horsetails), Lycopodiophyta (clubmosses, spikemosses, and quillworts), and Psilotophyta (whisk ferns). All of these phyla form antheridia and archegonia and produce free-swimming sperm, which require water to fertilize. Much like bryophytes, they reproduce with spores, but the sporophytes of these phyla are far more complex than those of the bryophyts, in that they have vascular tissue and well-differentiated leaves, roots, and stems.
These phyla are covered more extensively in the plant article.
Vascular Plant Structure
Plants have three major tissue systems: the vascular tissue system, the ground tissue system, and the dermal tissue system.
The vascular tissue system is comprised of the xylem and phloem. In general, the xylem conducts water and dissolved minerals, and the phloem conducts carbohydrates, primarily sucrose, for the plant to use as food. The phloem also conducts hormones, amino acids, and other substances necessary for plant growth.
Although the xylem is the principal water transporting medium and the phloem the main pathway of sugar transport, at times sugars do move in the xylem. An example of this is maple sap, used to produce maple syrup. In late winter/early spring in some maple trees, a sugary solution moves in the xylem, derived from carbohydrates stored in the stem. During cold nights, the hydrolysis of starch reserves in the xylem parenchyma cells produces sugars that are transported in the xylem during warm days, forced up the trunk by expanding carbon dioxide (CO2). These can be collected to produce maple syrup.
The ground tissue system consists of cubic or round cells with thin walls and living protoplasts, whose role is to aid in photosynthesis, storage, and secretion.
The dermal tissue system forms the outer protective covering for the plants, often consisting of cutin, a waxy substance forming the cuticle. Each of these three systems has its own series of specialized cells to aid in their respective function.
Vascular plant life cycle
In the first stage of development, the zygote, the first diploid cell of the plant, actively undergoes cell division, creating a mass of cells, known as the embryo. As the embryo develops, organs and tissues that make up the plant body begin to develop. At the root and shoot apices, meristems, or perpetually young tissues, establish, aiding the further development of these organs.
The differentiation of cells is almost immediate in angiosperms. The zygote divides with regard to its long axis, establishing polarity early on. The lower pole divides to produce the suspensor, which is involved in absorbing nutrients from the endosperm. The apical meristems first become obvious after about 6 days of cell division. The shoot meristem grows upward, differentiating leaves and branches, and the root meristem grows downward, differentiating root structure. These meristems remain active during the life cycle of the plant so that potentially uninhibited growth can proceed.
In gymnosperms, the zygote nucleus divides many times after fertilization, without forming a cell wall between each nucleus. After about eight rounds of cell division, about 256 nuclei occupy a large embryonic cell. Not until this time does cellular differentiation begin. The differential rate of cell division produces cells of different sizes, with the smaller ones being the result of faster division. In the larger cells, the microphyle develops into the suspensor. During this time the apical meristem also forms, giving rise to the root and then the shoot.
What differentiates embryogenesis of seed plants from other organisms is that cells remain in place the entire time, and the relation of one cell to another determines the fate and specialization of that cell.
Growth and development
Early in angiosperm embryonic development, an axis with either one or two cotyledons forms: monocots have one, whereas dicots have two. The cotyledon in monocots functions as the primary food absorbing organ. In dicots, the food is stored in the endosperm and maybe be absorbed into the cotyledons, which can become thick and fleshy, as in leguminous plants, or it may remain in the endosperm until seed maturity. Starches, fats, and oils are converted into sugars to feed the plant and allow for it to grow before it develops photosynthetic tissue. As the shoot develops, the leaves sprout above the cotyledon, in between which is the epicotyl, a portion of the stem axis that develops above the cotyledon. It can remain short and undifferentiated, or it can be longer and include one or more embryonic leaves. The epicotul together with the embryonic leaves is known as the plumule. The hypocotyl is the tissue below the cotyledon and the lower end of the hypocotyl is the radicle, which develops into the primary root. An enclosed plumule within a sheath is known as a coleoptile, and the radicle found inside the sheath is known as the coleorhiza. This is the case in grasses.
Because seeds can remain dormant until conditions are suited for growth, germination occurs just after the development of the plant outside of the seed coat, after the seed absorbs water, and metabolism begins. Initially this metabolism may be anaerobic, but aerobic activity soon takes over when the seed ruptures. Many seeds will not germinate until they have been stratified, held with a period below certain temperatures, to ensure that they do not sprout before the winter has passed. Metabolic reserves are stored in the amyloplasts, chloroplasts that are specialized to store starches. Cereal grains have a scutellum, a modified cotyledon, whose abundant food is used up first, after which the scutellum has the ability to absorb more food.
During the period from germination to the development of the seedling, the plant is highly susceptible to disease and drought, and is thus a critical period for the plant's survival.
Meristems may be either apical or lateral.
The most general form of meristem is the apical meristem (also called terminal meristem), found in buds at the tips of shoots, and at the root tip, and responsible for shoot and root growth, respectively. At the tip (apex) of the root, the apical meristem is covered and protected by a root cap of differentiated cells. Buds can be naked (with the growing leaves visible), protected by non-overlapping scales (valvate buds) or by overlapping scales (imbricated buds).
Apical meristems are completely undifferentiated (indeterminate). They may differentiate into three kinds of primary meristem:
- Protoderm lies around the outside of the stem, and develops into the epidermis.
- Procambium lies just inside the protoderm. It develops into the primary xylem, primary phloem. It also produces the vascular cambium, a secondary meristem that may continue to produce secondary xylem and phloem throughout the life of the plant.
- Ground meristem develops into the pith and the cork cambium, another secondary meristem.
The vascular cambium and cork cambium are called lateral meristems because they surround the established stem and make it grow larger in diameter. This is called the secondary growth, giving rise to wood. These plants are called arborescent or fruiticose. Secondary growth does not occur in all plants; those without it are called herbaceous.
Meristems located at a bud on a branch or shoot are known as a node. Tissue between nodes is known as the internode.
The epidermis (pluralized either epidermises or sometimes epidermes) is the outer single-layered group of cells covering the leaf and young tissues of a plant. It forms the boundary between the plant and the external world. The epidermis of most leaves shows dorsoventral anatomy: the upper (adaxial) and lower (abaxial) surfaces have somewhat different construction and may serve different functions.
The epidermis serves several functions: protection against water loss, regulation of gas exchange, secretion of metabolic compounds, and (in some species) absorption of water.
The epidermis is usually transparent (epidermal cells lack chloroplasts) and coated on the outer side with a waxy cuticle that prevents water loss. The cuticle may be thinner on the lower epidermis than on the upper epidermis; and is thicker on leaves from dry climates as compared with those from wet climates.
The epidermal tissue includes several differentiated cell types: epidermal cells, guard cells, subsidiary cells, and epidermal hairs (trichomes). The epidermal cells are the most numerous, largest, and least specialized. These are typically more elongated in the leaves of monocots than in those of dicots.
The epidermis is covered with pores called stomata (sing., stoma), part of a stoma complex consisting of a pore surrounded on each side by chloroplast-containing guard cells, and two to four subsidiary cells that lack chloroplasts. The stoma complex regulates the exchange of gases and water vapor between the outside air and the interior of the leaf. Typically, the stomata are more numerous over the abaxial (lower) epidermis than the (adaxial) upper epidermis.
Trichomes or hairs grow out from the epidermis in many species.
The stoma is bounded by two guard cells. The guard cells differ from the epidermal cells in the following aspects:
- The guard cells are bean-shaped in surface view, while the epidermal cells are irregular in shape.
- The guard cells contain chloroplasts, so they can manufacture food by photosynthesis. (The epidermal cells do not contain chloroplasts.)
- Guard Cells are the only epidermal cells that can make sugar. According to one theory, in sunlight the concentration of potassium ions (K+) increases in the guard cells. This, together with the sugars formed, lowers the water potential in the guard cells. As a result, water from other cells enter the guard cells by osmosis so they swell and become turgid. Because the guard cells have a thicker cellulose wall on one side of the cell (i.e. the side around the stomatal pore, the swollen guard cells become curved and pull the stomata open).
At night, the sugar is used up and water leaves the guard cells, so they become flaccid and the stomatal pore closes. In this way, they reduce the amount of water vapor escaping from the leaf. In vascular plants, the sporophytes are the dominant phase and the gametophytes are highly reduced.
The types of ground tissue found in plants develops from ground tissue meristem and consists of three simple tissues:
- Parenchyma (have retained their protoplasm)
- Collenchyma (have retained their protoplasm)
- Sclerenchyma (have lost their protoplasm in mature stage, i.e. are 'dead')
Parenchyma is the most common ground tissue. For example, it forms the cortex and pith of stems, the cortex of roots, the mesophyll (photosynthetic cells), the pulp of fruits, the endosperm of seeds, and the photosynthetic areas of a leaf. Parenchyma cells are capable of cell division even after maturation (i.e. they are still meristematic). They have thin, but flexible, cell walls, and are generally cube-shaped and are loosely packed. They have large central vacuoles, which allows the cells to store nutrients and water.
Parenchyma cells have a variety of functions;
- photosynthesis (may then be called Chlorenchyma /Mesophyll cells),
- gas exchange (Aerenchyma),
- secretion (e.g. Epithelial cells lining the inside of resin ducts)
- other specialized functions.
Collenchyma tissue is composed of elongated cells with unevenly thickened walls. They provide structural support, particularly in growing shoots and leaves. Collenchyma tissue composes, for example, the resilient strands in stalks of celery. Its growth is strongly affected by mechanical stress upon the plant. The walls of collenchyma in shaken plants may be 40 percent-100 percent thicker than those not shaken. The name collenchyma derives from the Greek word "kolla," meaning "glue," which refers to the thick, glistening appearance of the walls in fresh tissues.
There are three principal types of collenchyma;
- Angular collenchyma (thickened at intercellular contact points)
- Tangential collenchyma (cells arranged into ordered rows and thickened at the tangential face of the cell wall)
- Lacunar collenchyma (have intercellular space and thickening proximal to the intercellular space)
Sclerenchyma is a supporting tissue. Two groups of sclerenchyma cells exist: fibers and sclereids. Their walls consist of cellulose and/or lignin. Sclerenchyma cells are the principal supporting cells in plant tissues that have ceased elongation.
Unlike the collenchyma, mature sclerenchyma is composed of dead cells with extremely thick cell walls (secondary walls) that make up to 90 percent of the whole cell volume. The term "sclerenchyma" is derived from the Greek "scleros," meaning "hard". It is their hard, thick walls that make sclerenchyma cells important strengthening and supporting elements in plant parts that have ceased elongation. The difference between fibers and sclereids is not always clear. Transitions do exist, sometimes even within one and the same plant.
Fibers are generally long, slender, so-called prosenchymatous cells, usually occurring in strands or bundles. Such bundles or the totality of a stem's bundles are colloquially called fibers.
Many monocots have hard fibers. Typical examples are the fibers of many Gramineae, Agaves (sisal: Agave sisalana), lilies (Yucca or Phormium tenax), Musa textilis, and others. Their cell walls harbor, besides cellulose, a high proportion of lignin. The load-bearing capacity of Phormium tenax is as high as 20-25 kg/mm2 and is thus the same as that of good steel wire (25 kg/ mm2); however, the fiber tears as soon as too great a strain is placed upon it, while the wire distorts and tears not before a strain of 80 kg/mm2. The thickening of a cell wall has been studied in Linum. Starting at the center of the fiber are the thickening layers of the secondary wall deposited one after the other. Growth at both tips of the cell leads to simultaneous elongation. During development, the layers of secondary material seem like tubes, of which the outer one is always longer and older than the next. After completion of growth, the missing parts are supplemented, so that the wall is evenly thickened up to the tips of the fibers.
Meristematic tissues are usually the source of these fibers. Cambium and procambium are their main centers of production. They are often associated with the xylem of the vascular bundles. The fibers of the xylem are always lignified. Reliable evidence for the fiber cells' evolutionary origin of tracheids exists. During evolution, the strength of the cell walls was enhanced, the ability to conduct water was lost, and the size of the pits reduced. Fibers that do not belong to the xylem are bast (outside the ring of cambium) and such fibers are arranged in characteristic patterns at different sites of the shoot.
Sclereids are small bundles of sclerenchyma tissue in plants that form durable layers, such as the cores of apples and the gritty texture of pears. Sclereids are variable in shape. The cells can be isodiametric, prosenchymatic, forked, or fantastically branched. They can be grouped into bundles, can form complete tubes located at the periphery, or can occur as single cells or small groups of cells within parenchyma tissues. But compared with most fibers, sclereids are relatively short. Characteristic examples are the stone cells (called stone cells because of their hardness) of pears (Pyrus communis) and quinces (Cydonia oblonga), and those of the shoot of the wax plant (Hoya carnosa). The cell walls fill nearly all the cell's volume. A layering of the walls and the existence of branched pits is clearly visible. Branched pits such as these are called ramiform pits. The shell of many seeds, like those of nuts, as well as the stones of drupes like cherries or plums, are made up from sclereids.
- Mauseth, J. D. 2003. Botany: an introduction to plant biology. Jones and Bartlett Publishers. ISBN 0763721344
- Moore, R., W. Clark, W. Dennis, and D. S. Vodopich. 1998. Botany (3rd ed.). McGraw-Hill. ISBN 0697286231.
- Raven, P. H., and G. B. Johnson. 1996. Biology. (Fourth Edition). Wm.C. Brown Publishers. ISBN 0697225704